Quick answer
Коне́чно!
konéshna — spelled with ч, pronounced with ш
The options
Коне́чно!Of course! — pronounced “kanéshna”
Коне́чно, да.Yes, of course.
Разуме́ется.Naturally. — more formal
Ещё бы!You bet! — casual and emphatic
Без пробле́м.No problem.
The ч that sounds like ш
Two very common words break the rule: коне́чно → “kanéshna” and что → “shto”. Everywhere else ч is a normal ch.
What you hear back
Спаси́бо!Thanks!
Отли́чно!Great!
Договори́лись.Agreed.
Typical mistakes
Pronouncing коне́чно with a ch
“kanéshna”
One of the two fixed exceptions in Russian pronunciation.
Коне́чно нет — written without a comma
Коне́чно, нет.
коне́чно is set off by commas.
Check yourself
How is коне́чно pronounced?
Unstressed о reduces to a, and ч becomes ш.